From US v Miller as cited in Smith v Maryland:

"The depositor takes the risk, in revealing his affairs to another, that the information will be conveyed by that person to the Government. . . . This Court has held repeatedly that the Fourth Amendment does not prohibit the obtaining of information revealed to a third party and conveyed by him to Government authorities, even if the information is revealed on the assumption that it will be used only for a limited purpose and the confidence placed in the third party will not be betrayed."

US v Miller: http://scholar.google.com/scholar_case?case=1505272929564347...

Smith v Maryland: http://scholar.google.com/scholar_case?case=3033726127475530...